Make branch deletion URL more like GitHub's, fixes #1397 (#1994)

* Make branch deletion URL more like GitHub's, fixes #1397

* Add PR branch deletion integration test

* Do not allow deleting protected branch

* Change http error code to 403 if user has no write rights to repository

* Add check to not panic if forked repository has alrady been deleted

integrations/pull_create_test.go

@@ -7,6 +7,7 @@ package integrations
 import (
 	"net/http"
 	"path"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -21,6 +22,9 @@ func testPullCreate(t *testing.T, session *TestSession, user, repo, branch strin
 	htmlDoc := NewHTMLParser(t, resp.Body)
 	link, exists := htmlDoc.doc.Find("button.ui.green.small.button").Parent().Attr("href")
 	assert.True(t, exists, "The template has changed")
+	if branch != "master" {
+		link = strings.Replace(link, ":master", ":"+branch, 1)
+	}
 
 	req = NewRequest(t, "GET", link)
 	resp = session.MakeRequest(t, req)